Proteasome Inhibitors Prevent the Degradation of Familial Alzheimer’s Disease-Linked Presenilin 1 and Potentiate Aβ42 Recovery from Human Cells

Potentially Pathogenic SORL1 Mutations Observed in Autosomal-Dominant Cases of Alzheimer's Disease Do Not Modulate APP Physiopathological Processing

The SORL1 gene encodes LR11/SorLA, a protein that binds β-amyloid precursor protein (APP) and drives its intracellular trafficking. SORL1 mutations, occurring frequently in a subset of familial cases of Alzheimer's disease (AD), have been documented, but their pathogenic potential is not yet clear and questions remain concerning their putative influence on the physiopathological processing of APP. We have assessed the influence of two SORL1 mutations that were described as likely disease-causing and that were associated with either benign (SorLA924) or severe (SorLA511) AD phenotypes. We examined the influence of wild-type and mutants SorLA in transiently transfected HEK293 cells expressing either wild-type or Swedish mutated APP on APP expression, secreted Aβ and sAPPα levels, intracellular Aβ 40 and Aβ42 peptides, APP-CTFs (C99 and C83) expressions, α-, β- and γ-secretases expressions and activities as well as Aβ and CTFs-degrading enzymes. These paradigms were studied in control conditions or after pharmacological proteasomal modulation. We also established stably transfected CHO cells expressing wild-type SorLA and established the colocalization of APP and either wild-type or mutant SorLA. SorLA mutations partially disrupt co-localization of wild-type sorLA with APP. Overall, although we mostly confirmed previous data concerning the influence of wild-type SorLA on APP processing, we were unable to evidence significant alterations triggered by our set of SorLA mutants, whatever the cells or pharmacological conditions examined. Our study , however, does not rule out the possibility that other AD-linked SORL1 mutations could indeed affect APP processing, and that pathogenic mutations examined in the present study could interfere with other cellular pathways/triggers in AD.

Targeting amyloid clearance in Alzheimer's disease as a therapeutic strategy

Targeting the amyloid-β (Aβ) peptide cascade has been at the heart of therapeutic developments in Alzheimer's disease (AD) research for more than 25 years, yet no successful drugs have reached the marketplace based on this hypothesis. Nevertheless, the genetic and other evidence remains strong, if not overwhelming, that Aβ is central to the disease process. Most attention has focused on the biosynthesis of Aβ from its precursor protein through the successive actions of the β- and γ-secretases leading to the development of inhibitors of these membrane proteases. However, the levels of Aβ are maintained through a balance of its biosynthesis and clearance, which occurs both through further proteolysis by a family of amyloid-degrading enzymes (ADEs) and by a variety of transport processes. The development of late-onset AD appears to arise from a failure of these clearance mechanisms rather than by overproduction of the peptide. This review focuses on the nature of these clearance mechanisms, particularly the various proteases known to be involved, and their regulation and potential as therapeutic targets in AD drug development. The majority of the ADEs are zinc metalloproteases [e.g., the neprilysin (NEP) family, insulin-degrading enzyme, and angiotensin converting enzymes (ACE)]. Strategies for up-regulating the expression and activity of these enzymes, such as genetic, epigenetic, stem cell technology, and other pharmacological approaches, will be highlighted. Modifiable physiological mechanisms affecting the efficiency of Aβ clearance, including brain perfusion, obesity, diabetes, and sleep, will also be outlined. These new insights provide optimism for future therapeutic developments in AD research. LINKED ARTICLES: This article is part of a themed section on Therapeutics for Dementia and Alzheimer's Disease: New Directions for Precision Medicine. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.18/issuetoc.

Presenilins and γ-Secretase in Membrane Proteostasis

The presenilin (PS) proteins exert a crucial role in the pathogenesis of Alzheimer disease (AD) by mediating the intramembranous cleavage of amyloid precursor protein (APP) and the generation of amyloid β-protein (Aβ). The two homologous proteins PS1 and PS2 represent the catalytic subunits of distinct γ-secretase complexes that mediate a variety of cellular processes, including membrane protein metabolism, signal transduction, and cell differentiation. While the intramembrane cleavage of select proteins by γ-secretase is critical in the regulation of intracellular signaling pathways, the plethora of identified protein substrates could also indicate an important role of these enzyme complexes in membrane protein homeostasis. In line with this notion, PS proteins and/or γ-secretase has also been implicated in autophagy, a fundamental process for the maintenance of cellular functions and homeostasis. Dysfunction in the clearance of proteins in the lysosome and during autophagy has been shown to contribute to neurodegeneration. This review summarizes the recent knowledge about the role of PS proteins and γ-secretase in membrane protein metabolism and trafficking, and the functional relation to lysosomal activity and autophagy.

Contribution of the Endosomal-Lysosomal and Proteasomal Systems in Amyloid-β Precursor Protein Derived Fragments Processing

Aβ peptides, the major components of Alzheimer's disease (AD) amyloid deposits, are released following sequential cleavages by secretases of its precursor named the amyloid precursor protein (APP). In addition to secretases, degradation pathways, in particular the endosomal/lysosomal and proteasomal systems have been reported to contribute to APP processing. However, the respective role of each of these pathways toward APP metabolism remains to be established. To address this, we used HEK 293 cells and primary neurons expressing full-length wild type APP or the β-secretase-derived C99 fragment (β-CTF) in which degradation pathways were selectively blocked using pharmacological drugs. APP metabolites, including carboxy-terminal fragments (CTFs), soluble APP (sAPP) and Aβ peptides were studied. In this report, we show that APP-CTFs produced from endogenous or overexpressed full-length APP are mainly processed by γ-secretase and the endosomal/lysosomal pathway, while in sharp contrast, overexpressed C99 is mainly degraded by the proteasome and to a lesser extent by γ-secretase.

Twenty Years of Presenilins--Important Proteins in Health and Disease

Alzheimer's disease (AD) is characterized by progressive decline in cognitive functions associated with depositions of aggregated proteins in the form of extracellular plaques and neurofibrillary tangles in the brain. Extracellular plaques contain characteristic fibrils of amyloid β peptides (Aβ); tangles consist of paired helical filaments of the microtubuli-associated protein tau. Although AD manifests predominantly at ages above 65 years, rare cases show a much earlier onset of disease symptoms with very similar neuropathological characteristics. In 1995, two homologous genes were identified, in which mutations are associated with dominantly inherited familial forms of early onset AD. The genes therefore were dubbed presenilins (PS) and encode polytopic transmembrane proteins. At this time the role of these proteins in the pathogenesis of AD and their biological function in general were completely unknown. However, individuals carrying PS mutations showed alterations in the composition of different length variants of Aβ peptides in blood and cerebrospinal fluid, which indicated the potential involvement of presenilins in the metabolism of Aβ. After 20 years of intense research, the roles of presenilins in Aβ generation as well as important functions in biological processes have been identified. Presenilins represent the catalytic components of protease complexes that directly cleave the amyloid precursor protein (APP) but also many other proteins with important physiological functions. Here, the progress in presenilin research from basic characterization of their cellular functions to the targeting in clinical trials for AD therapy, and potential future directions, will be discussed.

Proteasome inhibition induces stress kinase dependent transport deficits--implications for Alzheimer's disease

Alzheimer's disease (AD) is characterized by accumulation of two misfolded and aggregated proteins, β-amyloid and hyperphosphorylated tau. Both cellular systems responsible for clearance of misfolded and aggregated proteins, the lysosomal and the proteasomal, have been shown to be malfunctioning in the aged brain and more so in patients with neurodegenerative diseases, including AD. This malfunction could be contributing to β-amyloid and tau accumulation, eventually aggregating in plaques and tangles. We have investigated the impact of decreased proteasome activity on tau phosphorylation as well as on microtubule stability and transport. To do this, we used our recently developed neuronal model where human SH-SY5Y cells obtain neuronal morphology and function through differentiation. We found that exposure to low doses of the proteasome inhibitor MG-115 caused tau phosphorylation, microtubule destabilization and disturbed neuritic transport. Furthermore, reduced proteasome activity activated several proteins implicated in tau phosphorylation and AD pathology, including c-Jun N-terminal kinase, c-Jun and extracellular signal-regulated protein kinase (ERK) 1/2. Restoration of the microtubule transport was achieved by inhibiting ERK 1/2 activation, and simultaneous inhibition of both ERK 1/2 and c-Jun reversed the proteasome inhibition-induced tau phosphorylation. Taken together, this study suggests that a decrease in proteasome activity can, through activation of c-Jun and ERK 1/2, result in several events related to neurodegenerative diseases. Restoration of proteasome activity or modulation of ERK 1/2 and c-Jun function can open new treatment possibilities against neurodegenerative diseases such as AD.

Review: unchained maladie - a reassessment of the role of Ubb(+1) -capped polyubiquitin chains in Alzheimer's disease

Molecular misreading allows the formation of mutant proteins in the absence of gene mutations. A mechanism has been proposed by which a frameshift mutant of the ubiquitin protein, Ubb(+1) , which accumulates in an age-dependent manner as a result of molecular misreading, contributes to neuropathology in Alzheimer's disease (Lam et al. 2000). Specifically, in the Ubb(+1) -mediated proteasome inhibition hypothesis Ubb(+1) 'caps' unanchored (that is, nonsubstrate linked) polyubiquitin chains, which then act as dominant inhibitors of the 26S proteasome. A review of subsequent literature indicates that this original hypothesis is broadly supported, and offers new insights into the mechanisms accounting for the age-dependent accumulation of Ubb(+1) , and how Ubb(+1) -mediated proteasome inhibition may contribute to Alzheimer's disease. Further, recent studies have highlighted a physiological role for free endogenous unanchored polyubiquitin chains in the direct activation of certain protein kinases. This raises the possibility that Ubb(+1) -capped unanchored polyubiquitin chains could also exert harmful effects through the aberrant activation of tau or other ubiquitin-dependent kinases, neuronal NF-κB activity or NF-κB-mediated neuroinflammatory processes.

PI3K inhibition causes the accumulation of ubiquitinated presenilin 1 without affecting the proteasome activity

gamma-Secretase is an enzymatic complex, composed of presenilin 1 (PS1), nicastrin, pen-2, and aph-1, and is responsible for the intramembranous cleavage of various type-I membrane proteins. The level of each component is tightly regulated in a cell via proteasomal degradation. On the other hand, it has previously been reported that PS1/gamma-secretase is involved in the activation of phosphatidylinositol-3 kinase/Akt (PI3K/Akt) pathway. PI3K is inhibited in Alzheimer's disease (AD) brain, whereas the effects of PI3K inhibition on the metabolism of PS1/gamma-secretase have not been elucidated. Here, we demonstrate that the treatment of neurons with PI3K inhibitors leads to increased levels of PS1/gamma-secretase components through an inhibitory effect on their degradation. Moreover, PI3K inhibition accelerated ubiquitination of PS1. We further show the evidence that the PS1 ubiquitination after PI3K inhibition is represented by the multiple mono-ubiquitination, instead of poly-ubiquitination. Accordingly, treatment of cells with PI3K inhibitor led to a differential intracellular redistribution of PS1 from the one observed after the proteasomal inhibition. These results suggest that PI3K inhibition may trigger the multiple mono-ubiquitination of PS1, which precludes the degradation of PS1/gamma-secretase through the proteasomal pathway. Since PS1/gamma-secretase is deeply involved in the production of Abeta protein, a deeper knowledge into its metabolism could contribute to a better elucidation of AD pathogenesis.

Pharmacological evidences for DFK167-sensitive presenilin-independent gamma-secretase-like activity

Amyloid-beta (Abeta) peptides production is thought to be a key event in the neurodegenerative process ultimately leading to Alzheimer's disease (AD) pathology. A bulk of studies concur to propose that the C-terminal moiety of Abeta is released from its precursor beta-amyloid precursor protein by a high molecular weight enzymatic complex referred to as gamma-secretase, that is composed of at least, nicastrin (NCT), Aph-1, Pen-2, and presenilins (PS) 1 or 2. They are thought to harbor the gamma-secretase catalytic activity. However, several lines of evidence suggest that additional gamma-secretase-like activities could potentially contribute to Abeta production. By means of a quenched fluorimetric substrate (JMV2660) mimicking the beta-amyloid precursor protein sequence targeted by gamma-secretase, we first show that as expected, this probe allows monitoring of an activity detectable in several cell systems including the neuronal cell line telencephalon specific murine neurons (TSM1). This activity is reduced by DFK167, N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (DAPT), and LY68458, three inhibitors known to functionally interact with PS. Interestingly, JMV2660 but not the unrelated peptide JMV2692, inhibits Abeta production in an in vitrogamma-secretase assay as expected from a putative substrate competitor. This activity is enhanced by PS1 and PS2 mutations known to be responsible for familial forms of AD and reduced by aspartyl mutations inactivating PS or in cells devoid of PS or NCT. However, we clearly establish that residual JMV2660-hydrolysing activity could be recovered in PS- and NCT-deficient fibroblasts and that this activity remained inhibited by DFK167. Overall, our study describes the presence of a proteolytic activity displaying gamma-secretase-like properties but independent of PS and still blocked by DFK167, suggesting that the PS-dependent complex could not be the unique gamma-secretase activity responsible for Abeta production and delineates PS-independent gamma-secretase activity as a potential additional therapeutic target to fight AD pathology.

Immunology and immunotherapy of Alzheimer's disease

Although Alzheimer's disease is considered to be a degenerative brain disease, it is clear that the immune system has an important role in the disease process. As discussed in this Review, immune-based therapies that are designed to remove amyloid-beta peptide from the brain have produced positive results in animal models of the disease and are being tested in humans with Alzheimer's disease. Although immunotherapy holds great promise for the treatment of Alzheimer's disease, clinical trials of active amyloid-beta vaccination of patients with Alzheimer's disease were discontinued after some patients developed meningoencephalitis. New immunotherapies using humoral and cell-based approaches are currently being investigated for the treatment and prevention of Alzheimer's disease.

Genetic association between Ubiquitin Carboxy-terminal Hydrolase-L1 gene S18Y polymorphism and sporadic Alzheimer's disease in a Chinese Han population

Increasing evidence indicates that the dysfunction of ubiquitin-proteasome system (UPS) is associated with Alzheimer's disease (AD). In the ubiquitin-proteasome pathway, Ubiquitin Carboxy-terminal Hydrolase-L1 (UCH-L1) plays an important role for the cellular clearance of abnormal proteins. Since a substitution of serine by tyrosine at codon 18, exon 3 (S18Y polymorphism) of the UCH-L1 gene exhibits a protective effect against the development of degenerative disease such as sporadic Parkinson's disease (PD) in several different ethnic groups, we hypothesized that UCH-L1 gene S18Y polymorphism may have that same effect on the pathologic process of AD. We examined UCH-L1 S18Y polymorphism genotypes of 116 sporadic AD patients and 123 healthy subjects in Chinese Han population using PCR-restriction fragment length polymorphism (RFLP) analysis. The allele and genotype data as well as data after stratification by age of onset failed to demonstrate any association between AD and S18Y polymorphism. However, after stratification by gender, female AD patients showed significantly less frequencies of Y allele and YY genotype in S18Y polymorphism than female controls (P = 0.003 and P = 0.015 respectively). We conclude that Y allele and YY genotype of S18Y in the UCH-L1 gene may have a protective effect against sporadic AD in female subjects, probably due to altering the function of UCH-L1 and the interactions among different risk factors.

Protein ubiquitination, degradation and the proteasome in neuro-degenerative disorders: no clear evidence for a significant pathogenetic role of proteasome failure in Alzheimer disease and related disorders

It has recently been proposed that Alzheimer disease (AD) might be initiated by a molecular 'hit' into a regulatory protein, e.g. a cell surface receptor [Schmitt HP. Neuro-modulation, aminergic neuro-disinhibition and neuro-degeneration: draft of a comprehensive theory for Alzheimer disease. Med Hypoth 2005;65:1106-19]. However, other substrates, in particular intra-cellular protein complexes such as the ubiquitin/proteasome system (UPS) could as well serve as a targets for such a 'hit' which might insert a mutation or induce conformational changes resulting in functional failure of protein degradation along the ubiquitin/proteasome proteolytic pathway. It has been claimed that impairment of the large multi-catalytic protease complex, the 20S/26S proteasome, might represent a key factor in the early pathogenesis of neuro-degenerative disorders characterized by the formation of abnormal protein aggregates such as neuronal cytoplasmic or nuclear inclusion bodies and fibrillary deposits. This article aims to review critically whether current information really supports the idea that impairment of the UPS might play a significant role in the early pathogenesis of neuro-degenerative disorders, with special emphasis on AD. The data provided in favour of proteasome impairment were, as a rule, revealed in in vitro experiments which cannot be unequivocally transferred to the in vivo conditions in neuro-degeneration. The author concludes that there is yet no clear evidence of a pivotal role of proteasome failure in the early pathogenesis of AD.

Lactacystin decreases amyloid-β peptide production by inhibiting β-secretase activity

The human amyloid precursor protein (APP) is processed by the nonamyloidogenic and the amyloidogenic catabolic pathways. The sequential cleavage of APP by the beta- and gamma-secretase activities, known as the amyloidogenic processing of APP, leads to the formation of the amyloid-beta peptide (Abeta). Abeta is the main constituent of the amyloid core of senile plaques, a typical hallmark of Alzheimer's disease. In addition to secretases, other cellular proteolytic activities, like the proteasome, might participate in the metabolism of APP. We investigated the consequence of proteasome inhibition on the amyloidogenic processing of human APP. CHO cells and primary cultures of rat cortical neurons expressing human APP or a protein corresponding to its beta-cleaved C-terminal fragment (C99) were treated with lactacystin, an irreversible inhibitor of the chymotrypsin-like activity of the proteasome. Lactacystin significantly decreased the level of Abeta produced from APP in both cellular models, whereas the production of Abeta from C99 was not affected. Lactacystin did not inhibit gamma-secretase activity but was found to inhibit the beta-cleavage of APP, leading to a proportional decrease in Abeta production. Although lactacystin did not inhibit the catalytic activity of recombinant BACE1, a decrease in neuronal beta-secretase activity was measured after treatment with lactacystin.

Proteasome-mediated effects on amyloid precursor protein processing at the gamma-secretase site

Abeta (beta-amyloid) peptides are found aggregated in the cortical amyloid plaques associated with Alzheimer's disease neuropathology. Inhibition of the proteasome alters the amount of Abeta produced from APP (amyloid precursor protein) by various cell lines in vitro. Proteasome activity is altered during aging, a major risk factor for Alzheimer's disease. In the present study, a human neuroblastoma cell line expressing the C-terminal 100 residues of APP (SH-SY5Y-SPA4CT) was used to determine the effect of proteasome inhibition, by lactacystin and Bz-LLL-COCHO (benzoyl-Leu-Leu-Leu-glyoxal), on APP processing at the gamma-secretase site. Proteasome inhibition caused a significant increase in Abeta peptide levels in medium conditioned by SH-SY5Y-SPA4CT cells, and was also associated with increased cell death. APP is a substrate of the apoptosis-associated caspase 3 protease, and we therefore investigated whether the increased Abeta levels could reflect caspase activation. We report that caspase activation was not required for proteasome-inhibitor-mediated effects on APP (SPA4CT) processing. Cleavage of Ac-DEVD-AMC (N-acetyl-Asp-Glu-Val-Asp-7-amino-4-methylcoumarin), a caspase substrate, was reduced following exposure of SH-SY5Y-SPA4CT cells to lactacystin, and co-treatment of cells with lactacystin and a caspase inhibitor [Z-DEVD-FMK (benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone)] resulted in higher Abeta levels in medium, augmenting those seen with lactacystin alone. This study indicated that proteasome inhibition could increase APP processing specifically at the gamma-secretase site, and increase release of Abeta, in the absence of caspase activation. This indicates that the decline in proteasome function associated with aging would contribute to increased Abeta levels.

Failure of the interaction between presenilin 1 and the substrate of gamma-secretase to produce Abeta in insect cells

Aggregates of beta-amyloid peptide (Abeta) are the major component of the amyloid core of the senile plaques observed in Alzheimer's disease (AD). Abeta results from the amyloidogenic processing of its precursor, the amyloid precursor protein (APP), by beta- and gamma-secretase activities. If beta-secretase has recently been identified and termed BACE, the identity of gamma-secretase is still obscure. Studies with knock-out mice showed that presenilin 1 (PS1), of which mutations are known to be the first cause of inherited AD, is mandatory for the gamma-secretase activity. However, the proteolytic activity of PS1 remains a matter of debate. Here we used transfected Sf9 insect cells, a cellular model lacking endogenous beta- and/or gamma-secretase activities, to characterize the role of BACE and PS1 in the amyloidogenic processing of human APP. We show that, in Sf9 cells, BACE performs the expected beta-secretase cleavage of APP, generating C99. We also show that C99, which is a substrate of gamma-secretase, tightly binds to the human PS1. Despite this interaction, Sf9 cells still do not produce Abeta. This strongly argues against a direct proteolytic activity of PS1 in APP processing, and points toward an implication of PS1 in trafficking/presenting its substrate to the gamma-secretase.

SEL-10 interacts with presenilin 1, facilitates its ubiquitination, and alters A-beta peptide production

Mutations in the human presenilin genes (PS1 or PS2) have been linked to autosomal dominant, early onset Alzheimer's disease (AD). Presenilins, probably as an essential part of gamma-secretase, modulate gamma-cleavage of the amyloid protein precursor (APP) to the amyloid beta-peptide (Abeta). Mutations in sel-12, a Caenorhabditis elegans presenilin homologue, cause a defect in egg laying that can be suppressed by loss of function mutations in a second gene, SEL-10. SEL-10 protein is a homologue of yeast Cdc4, a member of the SCF (Skp1-Cdc53/CUL1-F-box protein) E2-E3 ubiquitin ligase family. In this study, we show that human SEL-10 interacts with PS1 and enhances PS1 ubiquitination, thus altering cellular levels of unprocessed PS1 and its N- and C-terminal fragments. Co-transfection of sel-10 and APP cDNAs in HEK293 cells leads to an alteration in the metabolism of APP and to an increase in the production of amyloid beta-peptide, the principal component of amyloid plaque in Alzheimer's disease.

Generation of amyloid beta protein from a presenilin-1 and betaAPP complex

Presenilin-1 (PS1) is a causative gene in early onset familial Alzheimer's disease (FAD). FAD-linked mutant PS1s significantly increased Abeta40 and Abeta42(43) levels (P < 0.001) and decreased the production of an 11.4 kD (beta-stub) and an 8.7 kD (alpha-stub) carboxyl-terminal fragment of amyloid beta precursor protein (betaAPP-CTFs) (P < 0.01). In the 2% CHAPS extracted lysates, the complex containing the amino-terminal fragment of PS1 (PS1-NTF), the carboxyl-terminal fragments of PS1 (PS1-CTF), and betaAPP-CTFs was identified. Incubation of this isolated complex at pH 6.4 showed the direct generation of Abeta40 and gamma-stub from this complex. This reaction was inhibited by a gamma-secretase inhibitor. The degrading rate of a co-precipitated beta-stub was facilitated under the presence of FAD-linked mutant PS1s. This findings suggest that the direct generation of Abeta from the complex may play an important role in the pathogenesis of Alzheimer's disease.

The caspase-derived C-terminal fragment of betaAPP induces caspase-independent toxicity and triggers selective increase of Abeta42 in mammalian cells

During its physiopathological maturation, the beta-amyloid precursor protein undergoes several distinct proteolytic events by activities called secretases. In Alzheimer's disease, the main histological hallmark called senile plaque is clearly linked to the overproduction of the amyloid peptides Abeta40 and Abeta42, two highly aggregable betaAPP-derived fragments generated by combined cleavages by beta- and gamma-secretases. Recently, an alternative hydrolytic pathway was described, involving another category of proteolytic activities called caspases, responsible for the production of a 31 amino acids betaAPP C-terminal fragment called C31. C31 was reported to lower the viability of N2a cells but the exact mechanisms mediating C31-toxicity remained to be established. Here we show that the transient transfection of pSV2 vector encoding C31 lowers by about 80% TSM1 neuronal cells viability. Arguing against a C31-stimulated apoptotic response, we demonstrate by combined enzymatic and immunological approaches that C31 expression did not modulate basal or staurosporine-induced caspase 3-like activity and pro-caspase-3 activation. Furthermore, C31 did not modify Bax and p53 expressions, poly-(ADP-ribose)-polymerase cleavage and cytochrome c translocation into the cytosol. However, we established that C31 overexpression triggers selective increase of Abeta42 but not Abeta40 production by HEK293 cells expressing wild-type betaAPP751. Altogether, our data demonstrate that C31 induces a caspase-independent toxicity in TSM1 neurons and potentiates the pathogenic betaAPP maturation pathway by increasing selectively Abeta42 species in wild type-betaAPP-expressing human cells.

Glycogen synthase kinase-3beta regulates presenilin 1 C-terminal fragment levels

The majority of familial Alzheimer's disease cases have been attributed to mutations in the presenilin 1 (PS1) gene. PS1 is synthesized as an inactive holoprotein that undergoes endoproteolytic processing to generate a functional N- and C-terminal heterodimer (NTF and CTF, respectively). We identified a single residue in PS1, Ser(397), which regulates the CTF levels in a population of dimer that has a rapid turnover. This residue is part of a highly conserved glycogen synthase kinase-3beta (GSK-3beta) consensus phosphorylation site within the loop domain of PS1. Site-directed mutagenesis at the Ser(397) position increased levels of PS1 CTF but not NTF or holoprotein. Similar increases in only CTF levels were seen when cells expressing wild type PS1 were treated with lithium chloride, an inhibitor of GSK-3beta. Both wild type and PS1 S397A CTF displayed a biphasic turnover, reflecting rapidly degraded and stable populations. Rapid turnover was delayed for mutant PS1 S397A, causing increased CTF. These data demonstrate that PS1 NTF.CTF endoproteolytic fragments are generated in excess, that phosphorylation at Ser(397) by GSK-3beta regulates the discard of excess CTF, and that the disposal of surplus NTF is mediated by an independent mechanism. Overall, the results indicate that production of active NTF.CTF dimer is more complex than limited endoproteolysis of PS1 holoprotein and instead involves additional regulatory events.

The ubiquitin-proteasome pathway and proteasome inhibitors

The ubiquitin-proteasome pathway has emerged as a central player in the regulation of several diverse cellular processes. Here, we describe the important components of this complex biochemical machinery as well as several important cellular substrates targeted by this pathway and examples of human diseases resulting from defects in various components of the ubiquitin-proteasome pathway. In addition, this review covers the chemistry of synthetic and natural proteasome inhibitors, emphasizing their mode of actions toward the 20S proteasome. Given the importance of proteasome-mediated protein degradation in various intracellular processes, inhibitors of this pathway will continue to serve as both molecular probes of major cellular networks as well as potential therapeutic agents for various human diseases.

Does an inhibition of the ubiquitin/26S proteasome pathway of protein degradation underlie the pathogenesis of non-familial Alzheimer’s disease?

The ubiquitin/26S proteasome pathway catalyses the degradation of key regulatory proteins, as well as of misfolded or damaged polypeptides. Central to this pathway is the posttranslational covalent conjugation of ubiquitin to other eukaryotic target proteins, which acts as a signal for target protein degradation by the 26S proteasome proteolytic complex. Here, I propose a mechanism by which the expression of a frameshift ubiquitin mutant (termed 'ubiquitin(+1)'), which arises in the ageing human brain as a result of a process known as 'molecular misreading', could lead to a progressive age-dependent inhibition of the 26S proteasome. Further, I propose that such an inhibition contributes directly to Alzheimer's disease pathogenesis.

The regulation of presenilin-1 by nerve growth factor

Presenilin-1 (PS1) protein concentration is linked to neuronal development and to the pathogenesis of Alzheimer's disease, yet little is known about the biological factors and mechanisms that control cellular levels of PS1 protein. As PS1 levels are highest in the developing brain, we tested whether neurotrophin-induced differentiation influences PS1 expression using neuronotypic pheochromocytoma (PC12) cells. Treatment of PC12 cells with nerve growth factor (NGF) caused approximately 60-75% increases in the steady-state levels of endogenous PS1 N- and C-terminal fragments. PS1 protein accumulation was dose-responsive to NGF and required the presence of the TrkA NGF receptor tyrosine kinase. NGF also induced PS1 fragment accumulation in cultured explants of rat dorsal root ganglia. Quantitative northern blot analysis using PC12 cultures indicated that NGF did not increase steady-state PS1 mRNA levels. However, pulse-chase experiments indicated that NGF slowed the degradation rate of endogenous PS1 fragments, increasing the half-life from t(1/2) @22.5 to @25.0 h. This increase in half-life was insufficient to account for the approximately 60-75% increase in PS1 fragment levels measured in NGF-treated cells. Thus, NGF may regulate PS1 protein concentration in NGF-responsive cells by a complex mechanism that increases PS1 fragment production independent of holoprotein synthesis.

Inhibition of the ubiquitin-proteasome system in Alzheimer's disease

Alzheimer's disease is the most common cause of dementia in the elderly. Although several genetic defects have been identified in patients with a family history of this disease, the majority of cases involve individuals with no known genetic predisposition. A mutant form of ubiquitin, termed Ub(+1), has been selectively observed in the brains of Alzheimer's patients, including those with nonfamilial Alzheimer's disease, but it has been unclear why Ub(+1) expression should be deleterious. Here we show that Ub(+1) is an efficient substrate for polyubiquitination in vitro and in transfected human cells. The resulting polyubiquitin chains are refractory to disassembly by deubiquitinating enzymes and potently inhibit the degradation of a polyubiquitinated substrate by purified 26S proteasomes. Thus, expression of Ub(+1) in aging brain could result in dominant inhibition of the Ub-proteasome system, leading to neuropathologic consequences.

Presenilin 2 interacts with sorcin, a modulator of the ryanodine receptor

Perturbed Ca(2+) homeostasis is a common molecular consequence of familial Alzheimer's disease-linked presenilin mutations. We report here the molecular interaction of the large hydrophilic loop region of presenilin 2 (PS2) with sorcin, a penta-EF-hand Ca(2+)-binding protein that serves as a modulator of the ryanodine receptor intracellular Ca(2+) channel. The association of endogenous sorcin and PS2 was demonstrated in cultured cells and human brain tissues. Membrane-associated sorcin and a subset of the functional PS2 complexes were co-localized to a novel subcellular fraction that is distinctively positive for calcineurin B. Sorcin was found to interact with PS2 endoproteolytic fragments but not full-length PS2, and the sorcin/PS2 interaction was greatly enhanced by treatment with the Ca(2+) ionophore A23187. Our findings reveal a molecular link between PS2 and intracellular Ca(2+) channels (i.e. ryanodine receptor) and substantiate normal and/or pathological roles of PS2 in intracellular Ca(2+) homeostasis.

Alpha-synuclein and the Parkinson's disease-related mutant Ala53Thr-alpha-synuclein do not undergo proteasomal degradation in HEK293 and neuronal cells

Synucleins are neuronal proteins detectable in the neuropathological lesions of several cerebral disorders. Thus, alpha-synuclein immunoreactivity is found in Lewy bodies, the histopathological hallmark of sporadic Parkinson disease-affected brains. When mutated, alpha-synuclein seems to be responsible for some familial forms of Parkinson disease. As Lewy bodies are enriched in ubiquitinated structures and also contain proteasome-related immunoreactivity, it could be hypothesized that the proteasome contributes to the cellular degradation of alpha-synucleins, thereby controlling their concentration-dependent aggregation process. Here, we first demonstrate that alpha-synuclein is not ubiquitinated in HEK293 cells. Furthermore, by means of two specific inhibitors, we show that wild type and Ala53Thr alpha-synuclein do not behave as proteasome substrates in HEK293 cells and murine neurons. Our study indicates that the proteasome does not contribute to the control of cellular synucleins concentration and therefore, unlikely participates to cerebral alpha-synucleinopathies.

Mutational analysis of intrinsic regions of presenilin 2 that determine its endoproteolytic cleavage and pathological function

To investigate the significance of endoproteolytic processing of presenilin 2 (PS2) on its pathological function, we constructed PS2 cDNAs causing amino acid substitutions or deletions around the cleavage site. We found that a PS2 mutant (Del3) with a 20-amino acid deletion was not endoproteolytically processed, while other PS2s with amino acid substitutions and short deletions were cleaved. Overproduction of all the mutant proteins led to a compensatory decrease of endogenous PS1 fragments, but did not affect the amyloid beta peptide X-42/Abeta X-40 ratio without the familial Alzheimer's disease mutation. The Del3 mutant did not exhibit significant deficits in gamma-secretase activity. The turnover rate of the Del3 holoprotein was the same as that of full-length PS2. These data suggest that the determinants of the PS2 cleavage site reside within a large region and that the pathological function of PS2 is exerted by familial Alzheimer's disease mutations not related to the cleavage of holoproteins. We also found that PS2 with an 18-amino acid deletion at the C-terminal end was not processed. Its overexpression led neither to diminished accumulation of endogenous PS1 fragments nor to increased production of amyloid beta peptide X-42. The C-terminal end of PS2 seems to possess the signal for entry into the processing pathway.

Masking of an endoplasmic reticulum retention signal by its presence in the two subunits of the asialoglycoprotein receptor

Human asialoglycoprotein receptor H1 and H2b subunits assemble into a hetero-oligomer that travels to the cell surface. The H2a variant on the other hand is a precursor of a cleaved soluble form that is secreted. Uncleaved H2a precursor molecules cannot exit the endoplasmic reticulum (ER), a lumenal juxtamembrane pentapeptide being responsible for their retention. Insertion of this pentapeptide into H1 (H1i5) causes its complete ER retention but not fast degradation as happens to H2a. Cotransfection of H2a elicited, by heterodimerization, the Golgi processing of H1i5 and its surface expression. This occurred to a much lesser extent by cotransfection of H2b. Likewise, coexpression of H1i5 and not H1 stabilized H2a and caused its export to the cell surface. Homodimerization of molecules containing the pentapeptide did not cancel the retention. Thus, only when the pentapeptide is present in both subunits is the ER retention efficiently abrogated. The results show the unexpected finding that identical ER retention signals present in two associated chains can mask and cancel each other's effect. This could have important implications as similar abrogation of ER retention of other proteins could eventually be obtained by engineering and coexpressing an associated protein containing the same retention signal.

Alzheimer's disease associated presenilin 1 interacts with HC5 and ZETA, subunits of the catalytic 20S proteasome

Proteolytic processing and degradation tightly regulate the amount of stable, functional presenilin 1 (PSEN1) in the cell. The approximately 46-kDa PSEN1 holoprotein is cleaved into a approximately 30-kDa N-terminal fragment (NTF) and a approximately 20-kDa C-terminal fragment (CTF) by an unknown protease. The fragments are stabilized in a high molecular weight complex and nonincorporated fragments and excess holoprotein are degraded by the 26S proteasome. The tight balance between, on the one hand, processing and incorporation into the stable complex and, on the other hand, proteolytic degradation of excess PSEN1, indicates that minor changes in one of these two processes could be pathologically relevant. Here we demonstrate the direct physical interaction between PSEN1 and two subunits, HC5 and ZETA, of the 20S proteasome. These interactions were identified using an interaction trap screening and were further established in an in vitro binding assay. Furthermore, we were able to coimmunoprecipitate the transfected binding partners, as well as the endogenous PSEN1 and ZETA proteins from HEK 293T cells. Finally, degradation of ubiquitinated wild-type and mutant PSEN1 by the 26S proteasome was demonstrated. In conclusion, we report a direct interaction between PSEN1 and subunits of the 20S catalytic particle of the 26S proteasome, further establishing the involvement of proteasomal degradation in the regulation of PSEN1 turnover.

Processing of wild-type and mutant familial Alzheimer's disease-associated presenilin-1 in cultured neurons

Mutations of presenilin (PS)-1, an endoplasmic reticulum/Golgi transmembrane protein, have been associated with early-onset familial Alzheimer's disease (FAD). In mammalian brain, PS1 exists primarily as its processed fragments; however, the role of this cleavage event in PS1 function remains unclear. Although some investigators have shown that mutant PS1 processing is unaltered (with the exception of PS1-deltaE9, which lacks the cleavage site) in stably transfected cells and PS1-FAD transgenic mice, other investigators have reported altered FAD mutant PS1 and PS2 protein processing in transiently transfected cells and human FAD patients. The present study uses recombinant replication-defective adenoviral vectors to transiently express wild-type (WT) or mutant PS1 in various cells, including primary cultured hippocampal neurons. We show that in contrast to PS1-WT, overexpression of mutant PS1 results in an increased ratio of mutant holoprotein to endoproteolytic products that is dependent on cell type and differentiation state. In addition, mutant PS1 overexpression leads to an increase in caspase-type protease derived fragments above that seen with PS1-WT overexpression. Furthermore, overexpression of at least one mutant significantly alters the processing of coexpressed PS1-WT, suggesting that mutant PS1 may affect PS1-WT function. These findings suggest that a defect in PS1 holoprotein stability may be a general defect seen in cells expressing mutant PS1, especially neuronal cells, and may play a critical role in the pathogenesis of FAD.

Alzheimer's disease: correlation of the suppression of beta-amyloid peptide secretion from cultured cells with inhibition of the chymotrypsin-like activity of the proteasome

Peptide aldehyde inhibitors of the chymotrypsin-like activity of the proteasome (CLIP) such as N-acetyl-Leu-Leu-Nle-H (or ALLN) have been shown previously to inhibit the secretion of beta-amyloid peptide (A beta) from cells. To evaluate more fully the role of the proteasome in this process, we have tested the effects on A beta formation of a much wider range of peptide-based inhibitors of CLIP than published previously. The inhibitors tested included several peptide boronates, some of which proved to be the most potent peptide-based inhibitors of beta-amyloid production reported so far. We found that the ability of the peptide aldehyde and boronate inhibitors to suppress A beta formation from cells correlated extremely well with their potency as CLIP inhibitors. Thus, we conclude that the proteasome may be involved either directly or indirectly in A beta formation.

Calpain inhibitor I increases beta-amyloid peptide production by inhibiting the degradation of the substrate of gamma-secretase. Evidence that substrate availability limits beta-amyloid peptide production

The calpain inhibitor N-acetyl-leucyl-leucyl-norleucinal (ALLN) has been reported to have complex effects on the production of the beta-amyloid peptide (Abeta). In this study, the effects of ALLN on the processing of the amyloid precursor protein (APP) to Abeta were examined in 293 cells expressing APP or the C-terminal 100 amino acids of APP (C100). In cells expressing APP or low levels of C100, ALLN increased Abeta40 and Abeta42 secretion at low concentrations, decreased Abeta40 and Abeta42 secretion at high concentrations, and increased cellular levels of C100 in a concentration-dependent manner by inhibiting C100 degradation. Low concentrations of ALLN increased Abeta42 secretion more dramatically than Abeta40 secretion. ALLN treatment of cells expressing high levels of C100 did not alter cellular C100 levels and inhibited Abeta40 and Abeta42 secretion with similar IC50 values. These results suggest that C100 can be processed both by gamma-secretase and by a degradation pathway that is inhibited by low concentrations of ALLN. The data are consistent with inhibition of gamma-secretase by high concentrations of ALLN but do not support previous assertions that ALLN is a selective inhibitor of the gamma-secretase producing Abeta40. Rather, Abeta42 secretion may be more dependent on C100 substrate concentration than Abeta40 secretion.

Effect of protein kinase A inhibitors on the production of Abeta40 and Abeta42 by human cells expressing normal and Alzheimer's disease-linked mutated betaAPP and presenilin 1

1. We previously established that the formation of both alpha- and beta/gamma-secretase-derived products generated by human embryonic kidney 293 cells (HEK293) expressing either wild type or mutant betaAPP could be stimulated by agonists of the cyclic AMP/protein kinase A pathways. This cyclic AMP-dependent effect modulates post-translational events since it is not prevented by actinomycin D or cycloheximide. 2. We show here that two protein kinase A inhibitors, H89 and PKI, both trigger dose-dependent inhibition of the basal constitutive production of Abeta40 and Abeta42 by HEK293 cells expressing wild type betaAPP751. 3. H89 also potently inhibits the total Abeta produced by the neocortical neuronal cell line TSM1. 4. These two inhibitors also drastically reduce the recovery of Abeta40 and Abeta42 produced by HEK293 cells expressing the Swedish (Sw) betaAPP and M146V-presenilin 1 (PS1) mutations responsible for cases of the early-onset forms of Familial Alzheimer's disease (FAD). 5. By contrast, H89 and PKI do not significantly affect the recovery of the physiological alpha-secretase-derived fragment APPalpha. 6. Our study indicates that protein kinase A inhibitors selectively lower the formation of Abeta40 and Abeta42 in human cells expressing normal and mutant betaAPP and PS1 without affecting the physiological alpha-secretase pathway in these cells. Selective inhibitors of protein kinase A may be of therapeutic value in both sporadic and Familial Alzheimer's disease, since they may decrease the production of Abeta that is thought to be responsible for the neurodegenerative process.

Dual roles of proteasome in the metabolism of presenilin 1

Presenilin 1 (PS1) has been identified as a causative gene for most early-onset familial Alzheimer's disease. Biochemical studies revealed that PS1 exists predominantly as two processed fragments in cells and brain tissues. We prepared stably transfected cells expressing the wild-type and familial Alzheimer's disease-associated mutants of PS1 and investigated the enzyme that participates in the metabolism of PS1. After treatment of the cells with proteasome inhibitors, the full-length PS1 was significantly accumulated. The levels of N- and C-terminal fragments were also increased. The accumulation of PS1 with a deletion of exon 10, which is unable to be processed, on treatment of the transfected cells with lactacystin indicated that proteasome can degrade full-length PS1. A synthetic peptide that includes the processing region of PS1 was cleaved by 20S proteasome at the putative processing sites after Met288 and Glu299. Metabolic labeling experiments showed that the appearance of the N-terminal fragment was attenuated by the inhibitor. Finally, 28-kDa N- and 20-kDa C-terminal fragments were generated by purified PS1 in vitro. These data indicated that the proteasome pathway is involved in PS1 processing. These results demonstrate that the proteasome pathway plays dual roles in processing and degradation of PS1.

Evidence for functional and physical association between Caenorhabditis elegans SEL-10, a Cdc4p-related protein, and SEL-12 presenilin

Mutations in either of two human presenilin genes (PS1 and PS2) cause Alzheimer's disease. Here we describe genetic and physical interactions between Caenorhabditis elegans SEL-10, a member of the Cdc4p family of proteins, and SEL-12, a C. elegans presenilin. We show that loss of sel-10 activity can suppress the egg-laying defective phenotype associated with reducing sel-12 activity, and that SEL-10 can physically complex with SEL-12. Proteins of the Cdc4p family have been shown to target proteins for ubiquitin-mediated turnover. The functional and physical interaction between sel-10 and sel-12 therefore offers an approach to understanding how presenilin levels are normally regulated.

Expression of Alzheimer's disease-associated presenilin-1 is controlled by proteolytic degradation and complex formation

Numerous mutations causing early onset Alzheimer's disease have been identified in the presenilin (PS) genes, particularly the PS1 gene. Like the mutations identified within the beta-amyloid precursor protein gene, PS mutations cause the increased generation of a highly neurotoxic variant of amyloid beta-peptide. PS proteins are proteolytically processed to an N-terminal approximately 30-kDa (NTF) and a C-terminal approximately 20-kDa fragment (CTF20) that form a heterodimeric complex. We demonstrate that this complex is resistant to proteolytic degradation, whereas the full-length precursor is rapidly degraded. Degradation of the PS1 holoprotein is sensitive to inhibitors of the proteasome. Formation of a heterodimeric complex is required for the stability of both PS1 fragments, since fragments that do not co-immunoprecipitate with the PS complex are rapidly degraded by the proteasome. Mutant PS fragments not incorporated into the heterodimeric complex lose their pathological activity in abnormal amyloid beta-peptide generation even after inhibition of their proteolytic degradation. The PS1 heterodimeric complex can be attacked by proteinases of the caspase superfamily that generate an approximately 10-kDa proteolytic fragment (CTF10) from CTF20. CTF10 is rapidly degraded most likely by a calpain-like cysteine proteinase. From these data we conclude that PS1 metabolism is highly controlled by multiple proteolytic activities indicating that subtle changes in fragment generation/degradation might be important for Alzheimer's disease-associated pathology.